Although presently graphite is mainly used as a negative electrode material for a lithium ion secondary battery, it has been known that there exists a theoretical capacity limitation of 372 mAh/g in discharge capacity with respect to graphite. Since mobile devices, such as a cell phone, a notebook computer, and a tablet terminal, have come to have higher performance in recent years, a demand for a higher capacity lithium ion secondary battery has become stronger, and a negative electrode material, which can attain still higher capacity of a lithium ion secondary battery, has been sought-after.
Consequently, development of a negative electrode material containing an element, which has high theoretical capacity and ability for absorption and desorption of a lithium ion (hereinafter also referred to as “specific element”, and that containing the specific element is also referred to as “specific element substance”), has become active.
As the specific element, silicon, tin, lead, aluminum, etc. are well known. Among others, an oxide of silicon, which is one of the specific element substances, has advantages over a negative electrode material composed of other specific element substances, owing to higher capacity, lower cost, and better processibility, and negative electrode materials containing the same are especially energetically studied.
Meanwhile, the specific element substances are known to cause remarkable cubical expansion when alloyed by charging. Such cubical expansion micronizes a specific element substance itself, and further destroys the structure of a negative electrode material using the same, leading to a breakage of the electrical conductivity. Therefore it has a drawback in that the capacity decreases significantly over cycles.
With respect to the drawbacks, for example, Japanese Patent No. 3952180 discloses an electroconductive silicon complex for a negative electrode material for a nonaqueous electrolyte secondary battery, which is characterized in that a diffraction peak assignable to Si (111) is observed in X-ray diffraction, the crystal size of silicon determined by the Scherrer method based on the half width of the diffraction line is from 1 to 500 nm, and the surface of a particle having a structure where silicon crystallites are dispersed in a silicon compound is coated with carbon.
Japanese Patent No. 3952180 claims that the technology thereof can yield not only surface electroconductivity but also a structure stable against volume change due to absorption and release of lithium, and as the result improvement in long term stability and initial efficiency, by dispersing crystallites or fine particles of silicon in an inert rigid substance, for example, silicon dioxide, and fusing carbon over at least a part of the surface thereof for imparting electrical conductivity.
Japanese Patent No. 4171897 discloses a negative electrode material for a nonaqueous electrolyte secondary battery characterized in that the material is an electroconductive powder composed of a material which can absorb and release a lithium ion, and the surface of which is coated with a graphite film, and that the amount of the graphite coat is from 3 to 40 weight-%, the BET specific surface area is from 2 to 30 m2/g, and the graphite film shows spectra characteristic of a graphite structure near 1330 cm−1 and 1580 cm−1 of Raman shift in a Raman spectrum.
Japanese Patent No. 4171897 claims that the technology thereof can yield a negative electrode for a lithium ion secondary battery which can achieve a quality level demanded from the market, by regulating physical properties of a graphite film coated on the surface of a material, which can absorb and release a lithium ion, within a specific range.
Japanese Patent Application Laid-Open (JP-A) No. 2011-90869 discloses a negative electrode material for a nonaqueous electrolyte secondary battery, which is a negative electrode material to be used in a negative electrode for a secondary battery using a nonaqueous electrolyte, characterized in that the negative electrode material is composed of a particle of silicon oxide expressed by a general formula of SiOx whose surface is coated with a carbon film, and the carbon film is treated with a thermal plasma.
JP-A No. 2011-90869 claims that the technology thereof can yield a negative electrode material effective for a nonaqueous electrolyte secondary battery negative electrode, which has removed drawbacks of silicon oxide in expansion of an electrode and expansion of a battery by gas generation, and is superior in cycle performance.